A sliding nozzle apparatus is configured such that one of two or three refractory plates having a nozzle hole is slidingly moved while they are clamped at a high pressure (while they are applied with a surface pressure therebetween), to thereby change a degree of opening of the nozzle hole to control a flow rate of molten metal. This slidingly-movable plate (i.e., sliding plate) is held by a slide metal frame, which is provided in an openable and closable manner so as to enable the sliding plate to be replaced with a new one.
The sliding plate reaches its usable life after it is used only several times. Thus, there is a need to replace the sliding plate with a new one or check a damage state of the sliding plate, by opening the slide metal frame. In this case, it is necessary to release the surface pressure before opening the slide metal frame, and then apply the surface pressure again after closing the slide metal frame.
As a way to apply and release the surface pressure in the sliding nozzle apparatus, there has been known a technique of applying and releasing the surface pressure by means of sliding movement (sliding displacement) of the slide metal frame. That is, this technique is configured to cause a spring to be deformed by using a driving force during sliding movement of the slide metal frame. In this technique, a slide range (movable range) of the slide metal frame during an operation of applying or releasing the surface pressure is set to go beyond a slide range during a casting operation. Thus, in case of using two types of drive units (typically cylinder units) having different strokes between during the casting operation and during the surface pressure applying/releasing operation, there is a problem that it is necessary to additionally ensure a holding means and an installation space for a second, extra, one of the drive units.
On the other hand, there has also been proposed another technique of switchably changing the coupling position between the drive unit and the slide metal frame, by using one drive unit.
For example, the following Patent Document 1 discloses a coupling position switching mechanism configured to couple a drive unit and a slide casing (slide metal frame) through a guide piece, and switchably change a coupling position between the drive unit and the guide piece, within an opening provided in the guide piece by using a coupling pin. This guide piece is configured to be moved linearly based on a guide rail provided on a base frame, and an extension guide disposed to be slidingly moved along the guide rail in an extendable manner.
More specifically, as depicted in FIGS. 9 and 10, the guide piece 24 is provided with two, first and second, coupling holes 40, 41, whereby a coupling position between the guide piece 24 and a protruding portion 77e of a rod 7A serving as a coupling member can be switchably changed between during the casting operation and during the surface pressure applying/releasing operation, by selectively inserting a coupling pin 42 into one of the coupling holes 40, 41 and a coupling hole 77Aa formed in the protruding portion 77e. 
Further, as a means to position the protruding portion 77e, the guide piece 24 is formed with a first positioning surface a for use during the surface pressure applying/releasing operation, and a second positioning surface b for use during the casting operation. The first positioning surface a and the second positioning surface b are formed such that each of the first and second coupling holes 40, 41 and the coupling hole 77Aa of the protrusion portion 77e are coaxially arranged at each of the coupling positions, so as to enable the coupling pin 42 to be easily inserted thereinto and pulled out therefrom at each of the coupling portions.
However, in the coupling position switching mechanism disclosed in Patent Document 1, an operation of inserting and pulling out the coupling pin 42 is required every time the coupling position is switchably changed, thereby leading to a problem of deterioration in. efficiency of the switching operation
The coupling position switching mechanism disclosed in Patent Document 1 is also constructed such that the positioning surfaces a, b are provided in two areas within the opening of the guide piece 24 as described above so as to facilitate insertion and pull-out of the coupling pin 42. Further, the guide piece 24 is constructed such that a pair of the extension guides are provided bilaterally and slidingly moved, respectively, along a pair of the guide rails provided on the base frame, thereby leading to a problem that the coupling position switching mechanism becomes structurally complicated.
Moreover, the guide piece 24 is provided with a connection portion attachable and detachable with respect to the slide casing 4, thereby leading to a problem that the guide piece 24 is increased in size. Specifically, the guide piece 24 is subject to large stress under high temperatures during sliding movement. Thus, the connection portion attachable and detachable with respect to the slide casing 4 needs to be increased in size so as to be strengthened. For the same reason, a portion of the guide piece 24 defining the opening and a sliding surface also needs to be increased in size so as to be strengthened, so that a problem arises that the coupling position switching mechanism is increased in size and cost.